1. Field of the Invention
This invention relates generally to spatial power combiners and, more particularly, to such combiners which incorporate solid state-two-terminal negative resistance devices and which operate in the millimeter-wave and microwave frequency regions.
2. Description of the Prior Art
Despite the rapid progress made during the past years with the solid state millimeter-wave devices, the output power from a single device still falls short of the power required for some system applications, such as in radar and communication systems.
Even though the power output from a single device can still be improved through better design and better processing techniques, the attainable power from a single solid state device will ultimately be limited by some very fundamental device physics, such as maximum attainable power density and thermal considerations.
Power combining has long been recognized as a viable approach to overcome this obstacle. Several combining schemes under development in the past have yielded varying degrees of success. One such scheme is a chip-level combiner which has been successfully demonstrated at x-band with a good combining efficiency. Difficulties arise, however, when the chip configuration is scaled down for operation at higher frequencies (&gt;40 GHz). Control of mode stability and exessive RF losses seem to be the main problem. The device is very narrow band since the frequency of oscillation is mostly determined by the parasitic elements, which also hamper the optimization process. At very high frequencies, the number of chips that can be combined will be limited as the overall physical size begins to approach the characteristic wavelength. Consequently, the distributive nature and the possibility of the onset of the circuit modes can not be ignored.
Another scheme makes use of either resonant or non-resonant structure as a medium for power combining, and is generally known as a circuit-level combiner. For the resonant type, both cylindrical and rectangular waveguide resonator combiners have been extensively developed with impressive results. Notably, the Kurokawa-type rectangular resonator combiners have proven quite effective up to 217 GHz. Inherent limitations of a closed resonator combiner are its high mode density and narrow operational bandwidth. In addition, since practical packaged devices are finite in size, it becomes difficult to accommodate more than a few devices inside a resonator at high millimeter-wave frequencies without incurring serious moding problems and diode mutual coupling. Open resonant structures, such as a Fabry-Perot resonator, alleviate the problem associated with mode crowding considerably. However, little development work has been carried out in this area and no significant results are known.
As for non-resonant type circuit-level power combiners, both conical and radial line combining structures have been used in the microwave frequency range yielding some reasonable results. Since no resonant modes are involved in the combining process, these structures tend to have a wider operational bandwidth and are capable of accommodating a number of devices. Efforts to extend these technologies into millimeter-wave frequencies have not been met with comparable success due to difficulties in achieving the required mode stability and device-to-device isolation at these frequencies. Furthermore, the complexity required to implement mode filters and absorbing material within a millimeter-wave combiner structure renders these methods quite difficult.
Still another power combining scheme is known as a module-level combiner, and is characterized by the network-like topology to which the modules to be combined are connected and from which a resultant combined power is extracted. Since the modules can be single-device sources or combiners, the module-level combining can be used in conjunction with any combining methods described above including another module-level combiner.
Hybrid combiners employ the well known 3 db quadrature hybrid coupler as a basic combining component to form the combining network. The phasing requirement is accomplished by an appropriate circuit arrangement and an external injection lock. This method is rather straight-forward and has good operating bandwidth as well as good port-to-port isolation. However, as the number of modules to be combined increases, the hardware required along with the accompanying RF losses also increase until an optimal number is obtained beyond which the added power is offset by the RF losses. In practice, this optimal number is limited to 16 or less, depending on the frequency range. For the small number of power modules to be combined, this is a very low risk approach.
Another module-level combiner uses the N-way power divider/combiner technique which is very simple in principle, and is widely applied at lower microwave and RF frequencies. While the hybrid coupler combining scheme discussed above is more naturally applicable to one-port reflection type power modules (e.g., IMPATT sources) in the sense that circulators are not required, the N-way divider/combiner method requires separate input and ouput on the N-stages or chains of stages whose power is to be combined. Consequently, for IMPATT-type power modules, this scheme requires a large number of circulators to direct the power flow. It has, however, the important potential advantage of possessing truely graceful degradation characteristics in the event one or more of the combiner modules fails. To achieve this condition, the port-to-port isolation must be sufficiently high, which is difficult to achieve over a wide bandwidth for frequencies above approximately 40 GHz. For frequencies higher than 100 GHz, a Wilkinson-type N-way divider/combiner using dielectric waveguide seems promising, except that several key technologies, such as incorporating the circulators in a dielectric waveguide environment and establishing sufficient isolation among the combining parts, must be developed before this scheme can be realized.
In contrast to the above-described methods for combining the power of a plurality of power generating devices, the invention utilizes the spatial power combiner (SPC) technique which combines power through the space between a radiating aperture and a collecting aperture in a phased array where the desired phasing is achieved over the entire active radiating aperture simply and cheaply.